Study On Operating Characteristics Of Ablation-fed Pulsed Plasma Thrusters

As the first electric propulsion device for space application, pulsed plasma thruster(PPT) has several advantages such as high specific impulse, low power consumption, simple structure and convenient control. These advantages make PPT appropriate for a variety of missions including attitude control, station keeping and orbit rasing. Recently, PPT has become hot for studies and one of the most important research fields of propulsion system of micro-satellites.In order to improve the performance of the thruster and reduce the contamination of plume effectively, the operating characteristics of an ablation-fed pulsed plasma thruster(APPT) with a propellant of polytetrafluoroethylene(PTFE) have been studied theoretically, numerically and experimentally.A series of models for operation process, including discharge circuit model, two-phase ablation model, magnetohydrodynamic(MHD) model, thermochemical and transport model, have been established. The corresponding numerical algorithms have been proposed and a program has been developed, validated aiming at simulating the operation process of APPT. Studies on the ablation process of PTFE show that the mismatch of the ablation and discharge process, together with the oscillation of discharge current, decreases the thrust efficiency. Dividing the stored energy into two parts to ablate propellant and accelerate the plasma respectively allows for significant improvement of thrust performance. Rapid cutoff of current and propellant cooling at the end of the discharge may help to reduce the propellant mass losses due to particulate emission, thus enhancing the thrust efficiency. Numerical investigations on the inner flow field and the performance of APPT show that the plasma is accelerated and ejected from the electrode channel at a very high velocity under the strong electromagnetic force in the beginning of the discharge, producing a majority of the impulse bit. By contrast with this, in the end of the discharge the plasma moves downstream slowly since the gasdynamic force dominates the acceleration process, resulting in a lower thrust efficiency. In addition, influence of hall effect on the flow process of plasma in the electrode channel has been investigated. It shows that the hall effect would lead to an oblique flow towards the cathode side, generating a sideward thrust and causing a thrust loss.The damping characteristics of APPT current have been discussed. Results indicate that adjusting the discharge into a nonperiodic form by means of reducing circuit inductance or improving the design of discharge circuit would prolong the life of capacitors, enhance the efficiency of energy transfer and improve the electromagnetic acceleration effect. The performance comparison of different current waveforms has been done. It shows that the performance could be improved in a certain degree by applying a silicon stack in the discharge circuit, while a PFN circuit allows a much better performance.Considering the effect of magnetic field on the plasma plume, a hybrid particle-fluid model combining DSMC and PIC methods has been developed to study the plume characteristics of APPT. With inlet conditions obtained from the MHD results, this model has been applied to simulate the three-dimensional plume field. Investigations show that there are substantial differences between the dynamics of various species during the plume expansion process. It is indicated that the oscillation of discharge current leads to the generation of a low-velocity ion groups and the aggravation of ion backflow. It is demonstrated that electromagnetic acceleration is the primary mechanism of plasma acceleration, and the magnetic field plays an important role in the streamwise acceleration and backflow of the thruster plume. Furthermore, the results show that the contamination of plume in the perpendicular plane is severer compared to that in the parallel plane.A breech-fed rectangular APPT has been designed and manufactured. An improvement has been made in order to measure the micro-impulse of the thruster. Then the performance of the thruster has been evaluated based on the measurement of ablated mass and discharge parameters. The result indicates that the increase of the initial energy of capacitors could improve the performance of thruster effectively. Emission spectrum of the APPT discharges has been obtained by an optical fiber spectrometer. Results suggest the existence of the species of 2C,+C,2+C, F and +F. High speed photography of the discharge has been conducted in use of an ICCD camera and two bandpass filters. It indicates that a lot of plasma is accelerated to the cathode side in the operation process. During the discharge the ions are ejected from the electrode channel with a high velocity, followed by a large amount of neutral gas. These neutral particles move downstream slowly from the propellant surface, leading to a lower thrust efficiency of thruster. A triple Langmuir probe system and the corresponding data processing software based on Labview have been developed and implemented to obtain the values of plasma parameters in the plume. The diagnostic results show that there are two or even three peaks in curves of the temporal variation of the electron density and temperature during the discharge pulse. Generally the electron density and temperature decrease with increasing distance to the surface of propellant and increasing angle away from the centerline of thruster. The distribution of plume is inclined towards the cathode in the perpendicular plane, while much more symmetric in the parallel plane.